Waveguide to microstrip coupler

ABSTRACT

In a waveguide to microstrip coupler comprising a metal part fastened to one of the waveguide large walls in which a channel is machined and a strip line within said channel protruding through an opening in said waveguide large wall, the distance between the nonmetalized face of the strip line substrate and the channel wall is set to a value depending on the mid band frequency of the transmitted bandwidth in order to provide a bandwidth equal to 40 percent of the mid band frequency value.

United States Patent Fache et al.

Dec. 2, 1975 7 IIIIIIIIIIIIIIIIIJIIIIIIIIIlIllIlI/III:

Primary E.raminer-Paul L. Gensler Attorney, Agent, or Firm-Kemon, Palmer & Estabrook ABSTRACT In a waveguide to microstrip coupler comprising a metal part fastened to one of the waveguide large walls in which a channel is machined and a strip line within said channel protruding through an opening in said waveguide large wall. the distance between the nonmetalized face of the strip line substrate and the channel wall is set to a value depending on the mid band frequency of the transmitted bandwidth in order to provide ahbandwidth equal to 40 percent of the mid band frequency value.

3 Claims, 5 Drawing Figures jlllllllllll l llnlllllllllllll [II II 'HTlTrIl'llIlTlillllllllllllllI/l US Patent Dec. 2, 1975 Sheet i of; 3,924,204

PRIOR ART J JIIIIIIJII l [III[lIIIIIIIIIIIIIIIIILIIIIllIIr17111/ JIIIIIIIIIIIIIII I I II 77 'IIIII'TIIIII IMIITII'II[MIL]??? I wt IIIIIINIIIIIIII WAVEGUIDE TO MICROS'IRIP COUPLER BACKGROUND OFTH E INVENTION. PRIOR ART The inventionrelates to an improvement in wideband coupling arrangements between a waveguide and a microstrip line terminated by an antenna within the waveguide. 4

US. Pat. No. 3,462,713, filed by the BELL TELE- PHONE LABORATORIES INCORPORATED on the July 19th 1967 and entitled Waveguide stripline transducer discloses inits introduction, a coupler, represented in FIG. 1, between a rectangular waveguide and a microstrip line 11, situated in the mean plane of the large walls of the waveguide perpendicular to said walls. In this coupler, the wall of the rectangular guide shows a small slot opening into a rectangular section channel, of like section, extending completely through a metal body 13 connected to the waveguide 10. The microstrip line 11 is formed by a dielectric substrate 14, a printed conductor or strip 12 and the large wall of the channel formed in body 13, on which lies the dielectric l4. The micros trip could also be a three plate line. In such a line, the'conductor '12 is positioned between two dielectric substratesfilling the channel and the two earth planes are formed by the inner parts of the two large walls of the -said channel. The conductor 12 protrudes into the waveguide 10 and is terminated by an antenna 16 with stubssuch as 17 and 18. A coupler of this type, made of a channel with a longitudinal axis perpendicular to a large wall of a rectangular guide and in which the said channel houses and insulating substrate carrying a strip conductor protruding into the guide through an opening in said large wall will be hereinafter called a coupler of the above type.

BRIEF DISCLOSURE OF THE INVENTION The object of the present invention is to provide a means of achieving a waveguide-microstrip line coupler, of which the relative bandwidth is at least equal to 40 percent and the production of which does not require any high precision machining step in order that the reproducibility of the coupler characteristics be assured.

The waveguide-microstrip line coupler with a relative bandwidth at least equal to 40 percent, centered on the frequency f according to the invention, is a coupler of the above type, comprising: a channel machined in a metal body the plane of symmetry of said channel coinciding with that of the large walls of the waveguide and the dimensions of which are such that the cut-out frequency of a TE wave being propagated therein is higher than the maximum frequency of the band to be transmitted and a microstrip line formed of a dielectric substrate carrying a conductive strip on a first of its large' faces, the second large face of which is packed on two metallic bars fast with one of the large walls of the said channel.

According to an important feature of the invention, the thickness e of the said packing pieces, expressed in meters, is at least equal to 5.10 cf where c being the speed of light and f the middleband frequency in hertz, which is equivalent to e 2 1.5 X 10 f DETAILED DESCRIPTION OF THE INVENTION As an illustration, a coupler has been developed according to the invention, having a standing wave ratio (26.5 to 40 GHz), with an insertion loss smaller than 0.3 dB. A coupler with a standing wave ratio smaller than 1.3 has also been designed, covering the whole K- band (12 to 18 GI-lz). In both cases, the relative bandwidth is 40 percent.

Other features and advantages of the invention will become apparent from the description and by reference to FIGS. 2 to 5, given simply by way of illustration and without being .limitative in any way, and in which:

FIG. 1 is aperspective view partially in section of-a prior art coupler FIG. 2 is a sectional view of the coupler according to the invention through a planewhich is medial to the large. wallsof the waveguide;

FIG. 3 is a sectional view of the channel fixed laterally to the waveguide 'through the plane shown at A -A in FIG. 2;

FIG. 4 represents thevariation in the standing wave ratio of a waveguide to microstrip line coupler as a function of the frequency for several thicknesses of the packing pieces;

FIG. 5 represents the variation curves of the standing wave ratio of the coupler as a function of the frequency for two depth of penetration of the microstrip line into the waveguide. V I

FIG. 2 represents a waveguide 21 and rectangular substrate 22 of a low loss dielectric in the millimetric frequency band ,v penetrating into the waveguide through a slot; the upper part of 22 extends through a channel 23 of rectangular section form in a metal body 24 fixed to the waveguide by any known means. One of the faces of this substrate supports a conductor or strip 25, designed according to any one of the processes used at present in the thick film technique, in the pattern of a narrow rectilinear ribbon. The strip 25 is terminated before the end of the substrate 22 which extends into the waveguide 21 in abutment with the opposite wall. The part of the strip 25 penetrating into the guide 21 forms a rectilinear antenna situated at approximately one quarter wave from the short-circuit plane 26. The dielectric constituting the substrate can be quartz, alumina of the quality usually used as a substrate for microwave integrated circuits or even nickel ferrite with low microwave losses.

The rectangular channel 23 with a longitudinal axis perpendicular to the large sides of the waveguide 21 has large walls parallel to the longitudinal axis of the latter. The length of these sides is sufficiently small so that no guided mode within the frequency band propagated through the waveguide 21 can be transmitted along the channel 23 without considerable attenuation. The length of the channel 23 is sufficient so that, at its output end the attenuation of the guided modes be higher than a preset value, 20 dB for example. Under these conditions, the only mode which is propagated without attenuation is the TEM mode in the microstrip line 22-25. The wall 29 of the channel 23 carries a frame 28-28 against which the substrate 22 is placed.

FIG. 3 represents a sectional view in a plane perpendicular to that of FIG. 2 and cutting the latter along the line AA. The width of the frame 28 and 28' along the wall 29 is so chosen that the bandwidth of the coupler is maximum and the S.W.R. is smaller than 1.3. The thickness e of the frame 28 and 28' above the channel wall 29 is chosen as follows. All other things being equal when the value of e is caused to increase from zero, it has been remarked that the lack of regularity of the variation curve of the S.W.R. as a function of the frequency diminishes to begin with. By way of illustration, FIG. 4 represents at 40 a curve which is typical of the variation of the S.W.R. of a coupler between a RG waveguide 96/U and a microstrip with e 0. When e reaches the value 1.5 X 10 f the S.W.R. shows no longer any irregularity, as shown in curve 41 corresponding to e 5.10 meters. A slight supplementary increase in e does not produce any substantial improvement, as is shown by the curve 42 relating to a thickness e 10 m. Starting from this value, any further increase of e causes a deterioration of the curve. All other things being equal, it was established that the width of the frame has no effect, at least as long as it remains smaller than one tenth of the length of the large dimension of the channel.

The length of conductor 25 within the waveguide 21 is close to one quarter wavelength. However, an experimental adjustment of this length makes it possible either to obtain a coupler with a large bandwidth, and a S.W.R. smaller than 1.3, or to design a coupler with a narrower bandwidth and a S.W.R. very close to 1.

FIG. 5 represents two variation curves of the S.W.R. of the coupler, taken as an example for illustrative purposes, between a RG waveguide 96/U and a microstrip line, in which the frame 28 and 28' is 5.10 m thick. The curve 50 relates to a conductor 25 penetrating for 1.6 X 10 in into the waveguide, while the curve 51 relates to a conductor 25 penetrating for 1.25 X 10 m into the waveguide.

What we claim:

1. A broadband coupler between a rectangular waveguide and a microstrip line comprising a rectangular channel formed in a metal body fixed on a first large wall of said waveguide and perpendicular thereto with the largest dimension of said channel parallel to the axis of said waveguide, a rectangular dielectric substrate plate within said channel extending into said waveguide through a slot in said first wall up to the second large wall, a conductor printed on a first face of said dielectric plate extending into said waveguide, in which packing means are provided on one inner wall of said channel to support the second face of said dielectric plate so that the distance e between the inner wall and said second face of said plate has a range of where:

f is the midband frequency in hertz of the operating frequency band and e is measured in meters.

2. A broadband coupler according to claim 1 in which said packing means is a frame carried by the inner wall of said channel of thickness e and of width along the inner wall less than l/ 10 of the largest dimension of said channel cross-section.

3. A broadband coupler according to claim 1 in which said dielectric plate is made of a low loss nickel microwave ferrite. 

1. A broadband coupler between a rectangular waveguide and a microstrip line comprising a rectangular channel formed in a metal body fixed on a first large wall of said waveguide and perpendicular thereto with the largest dimension of said channel parallel to the axis of said waveguide, a rectangular dielectric substrate plate within said channel extending into said waveguide through a slot in said first wall up to the second large wall, a conductor printed on a first face of said dielectric plate extending into said waveguide, in which packing means are provided on one inner wall of said channel to support the second face of said dielectric plate so that the distance ''''e'''' between the inner wall and said second face of said plate has a range of values defined by 3 X 106/f > e > OR = 1.5 X 106/f where: f is the midband frequency in hertz of the operating frequency band and e is measured in meters.
 2. A broadband coupler according to claim 1 in which said packing means is a frame carried by the inner wall of said channel of thickness ''''e'''' and of width along the inner wall less than 1/10 of the largest dimension of said channel cross-section.
 3. A broadband coupler according to claim 1 in which said dielectric plate is made of a low loss nickel microwave ferrite. 